1. Field of the Invention
The present invention relates to the art of taking measures against heat build-up in a discharge-lamp lighting circuit.
2. Description of the Related Art
There is a known discharge-lamp lighting circuit configuration comprising a DC power supply circuit, a DC-AC converter circuit (DC-DC converter) and a starting circuit (a so-called starter circuit). In such a circuit configuration using a DC-DC converter circuit in a DC power supply circuit, for example, a full-bridge type circuit (a circuit for performing on-off control by using two sets of four semiconductor switching elements) with a driver circuit is employed as the DC-AC converter circuit, wherein the output voltage of the DC-DC converter circuit is converted to rectangular-wave voltage in the full-bridge type circuit before being supplied to a discharge lamp.
In consideration of applying the aforementioned art to automotive discharge lamps, it has been known to perform control (so-called derating control) over a lowering of output power to be supplied to discharge lamps in proportion to a voltage drop when input voltage from a power supply lowers in case where DC power supplies (batteries) are employed in lighting circuits. In this case, the purpose is for only preventing the exhaustion of the battery but also protecting the lighting circuit. In the case of the latter, battery current (input current) tends to increase in case where control works to maintain the supply of prescribed power though the battery voltage is lowering. Consequently, there is the fear of causing loss to increase and electric conversion efficiency (ratio of the output power to the input power) to worsen. Incidentally, the worsening of the conversion efficiency results in increasing loss as the battery current increases further and when this vicious cycle is accelerated, the generation of heat in the circuit and the heat destruction thereof may occurs if the worst to the worst.
Therefore, as a countermeasure, it is necessary for performing a control to lower the supply of the output power to the discharge lamp in proportion to the lowering of the application of the input voltage to the lighting circuit and the following forms are known, for example.
(1) A form of detecting and monitoring the battery current using current detecting elements (e.g., shunt resistors) so as to limit the current to the extent that its detected value is a predetermined current value or less.
(2) A form of limiting currents flowing through switching elements constituting a DC-DC converter (e.g., a pulse by pulse current limiting method).
(3) A form of lowering the supply of power to the discharge lamp when the lowering of the battery voltage is detected while the batter voltage is being monitored.
In the forms (1) and (2), such heat destruction is prevented by observing the current value directly concerned with the generation of heat in the circuit to limit the current so that no excessive current is allowed to flow into the current, which results in lowering the supply of power to the discharge lamp.
With regard to the form (3), the output power may be controlled in accordance with the characteristics conceptually shown in FIG. 7, for example, wherein the battery voltage (referred to xe2x80x98Vinxe2x80x99) is taken on the horizontal axis, whereas the supply of power (referred to xe2x80x98PWxe2x80x99) to the discharge lamp is taken on the vertical axis, whereby control characteristics are shown by a graphic line g.
The flat portion of the graphic line g indicates the rated power of the discharge lamp and in case where the input voltage Vin from the battery is within tolerance, rated power is supplied to the discharge lamp while the discharge lamp is steadily lighted. However, the supply of power PW is so controlled that the power is lowered as the value of Vin decreases (see the tilted portion of the graphic line g).
Nevertheless, unsatisfactory measures have been taken to counter the generation of heat in circuits as well as the heat destruction of circuits according to the conventional methods and the following inconvenience may be caused.
On the assumption that lighting circuits are used in cold districts (the ambient temperature is low), though there is still a margin for a change in the temperature until circuit failure occurs, the derating function for decreasing the supply of power to a discharge lamp inevitably works as input voltage lowers and it is feared that the brightness of the discharge lamp grows weaker than what is prescribed. Conversely, a margin for a change in the temperature falls because the ambient temperature rises (e.g., due to the generation of heat from the engine of a vehicle) and when danger of circuit failure increases, it is still feared that a current limiting function for sufficiently preventing heat build-up and a power lowering function are not demonstrated.
A problem to be solved by the invention is to take satisfactory measures to counter the generation of heat in a discharge-lamp lighting circuit in consideration of the ambient temperature.
In order to solve the foregoing problems, a discharge-lamp lighting circuit according to the invention comprises a DC-DC converter circuit for boosting or lowering DC input voltage from a DC power supply, a DC-AC converter circuit for converting the output voltage of the DC-DC converter circuit to AC voltage, and a control circuit for controlling the supply of electric power to a discharge lamp, whereby the supply of power to the discharge lamp is controlled by the control circuit in response to the lowering of the DC input voltage when the lowering of the DC input voltage is detected, wherein power control is performed so that the supply of power to the discharge lamp is decreased by the control circuit as the ambient temperature rises even though the lowering of the DC input voltage remains unchanged.
According to the invention, the supply of power to the discharge lamp decreases as the ambient temperature rises when the DC input voltage lowers even though the lowering of the DC input voltage remains unchanged, whereby satisfactory measures can be taken to counter heat build-up by suppressing the generation of heat in the circuit due to the ambient temperature rise. Therefore, it is possible to avoid the inconvenience caused by decreasing the supply of power to the discharge lamp more than necessary when the ambient temperature is low.